A lot of energy is wasted on flexing of the panels.
If you can strengthen up those panels and redirect the energy, the net results will be an increase in spl
Think about it like this:
Lets say you put a 10 inch sub woofer in 2 different sealed enclosures and send 500 watts to each sub.
The first enclosure is made out of cardboard. The entire things is nothing more than a UPS box that is strong enough to support the weight of the sub. Cardboard.
The other box is 3/4" mdf, braced, and sealed inside and out side with Rhino Liner.
You could imagine the difference between the 2 boxes. The energy would be contained in the mdf enclosure, and would exit where it is supposed to, out of the sub.
The first box however would be very flimsy, and would not do a very good job at containing the energy. Energy (sound pressure) would escape everywhere, including the sub. The net results would be less bass.
This is sort of what you are doing by deadening your car. You are strengthening the panels, reducing the flex, killing vibrations and the noise that goes with them, and generally giving your audio system a more stable environmet to perform in.
Hope that helps